Ga2O3-based single crystal substrate, and production method therefor

ABSTRACT

Provided are a Ga 2 O 3 -based single crystal substrate including a Ga 2 O 3 -based single crystal which has a high resistance while preventing a lowering of crystal quality and a production method therefor. According to one embodiment of the present invention, the production method includes growing the Ga 2 O 3 -based single crystal while adding a Fe to a Ga 2 O 3 -based raw material, the Ga 2 O 3 -based single crystal ( 5 ) including the Fe at a concentration higher than that of a donor impurity mixed in the Ga 2 O 3 -based raw material, and cutting out the Ga 2 O 3 -based single crystal substrate from the Ga 2 O 3 -based single crystal ( 5 ).

TECHNICAL FIELD

The invention relates to a Ga₂O₃-based single crystal substrate and aproduction method therefor.

BACKGROUND ART

A method is known in which Mg, Be or Zn is doped so as to increase theresistivity of a Ga₂O₃ single crystal (see e.g., PTL 1). PTL 1 statesthat the resistivity of the Ga₂O₃ single crystal can be increased byadding 0.01 mol % or 0.05 mol % of Mg in growing the Ga₂O₃ singlecrystal by a FZ (Floating Zone) method.

CITATION LIST Patent Literature

[PTL 1]

JP-A-2011-102235

SUMMARY OF INVENTION Technical Problem

As a raw material of Ga₂O₃ single crystal, Ga₂O₃ powder having a purityof not more than 99.999 mass % is widely used. It is technicallypossible to produce Ga₂O₃ powder having a higher purity but it is notrealistic in terms of cost. Ga₂O₃ powder having a purity of not morethan 99.999 mass % contains a trace amount of Si (donor impurity) as aresidual impurity and a Ga₂O₃ single crystal grown using such Ga₂O₃powder exhibits n-type conductivity. The concentration of Si included inthe Ga₂O₃ powder has a distribution in the Ga₂O₃ single crystal. Forexample, the Ga₂O₃ single crystal grown using Ga₂O₃ powder having apurity of 99.999 mass % as a raw material has a Si concentration ofabout 5×10¹⁷ cm⁻³ at the most highly concentrated portion.

Therefore, in order to manufacture a high-resistivity Ga₂O₃ substrate,the Ga₂O₃ single crystal needs to be doped with an acceptor impurity ata concentration of at least 5×10¹⁷ cm⁻³ or more. If cheaper low-purityGa₂O₃ powder is used as a raw material of Ga₂O₃ single crystal, it isnecessary to dope the acceptor impurity at a higher concentration.

Generally, in doping a high-concentration impurity into a singlecrystal, a problem may arise that it is difficult to dope an impurity ata target concentration and that the crystal quality of the singlecrystal decreases due to the doping.

Thus, it is an object of the invention to provide a Ga₂O₃-based singlecrystal substrate comprising a Ga₂O₃-based single crystal that has ahigh resistance and while preventing a lowering of crystalline quality,as well as a production method therefor.

Solution to Problem

According to one embodiment of the invention, a production method for aGa₂O₃-based single crystal substrate set forth in [1] to [5] below isprovided so as to achieve the above object.

[1] A production method for a Ga₂O₃-based single crystal substrate,comprising:

-   -   a step of growing a Ga₂O₃-based single crystal while adding a Fe        to a Ga₂O₃-based raw material, the Ga₂O₃-based single crystal        comprising the Fe at a concentration higher than that of a donor        impurity mixed in the Ga₂O₃-based raw material; and    -   a step of cutting out the Ga₂O₃-based single crystal substrate        from the Ga₂O₃-based single crystal.

[2] The production method for a Ga₂O₃-based single crystal substrateaccording to [1],

-   -   wherein the Ga₂O₃-based raw material has a purity of 99.999 mass        %, and    -   wherein the Ga₂O₃-based single crystal grown comprises the Fe at        a concentration of not less than 5×10¹⁷ cm⁻³.

[3] The production method for a Ga₂O₃-based single crystal substrateaccording to [1],

-   -   wherein the Ga₂O₃-based raw material has a purity of 99.99 mass        %, and    -   wherein the Ga₂O₃-based single crystal grown comprises the Fe at        a concentration of not less than 5×10¹⁸ cm⁻³.

[4] The production method for a Ga₂O₃-based single crystal substrateaccording to any one of [1] to [3], wherein the donor impurity comprisesSi.

[5] The production method for a Ga₂O₃-based single crystal substrateaccording to any one of [1] to [3], wherein a principal surface of theGa₂O₃ single crystal substrate has a size and a shape large enough toinclude a perfect circle of not less than 10 mm in diameter.

According to another embodiment of the invention, a Ga₂O₃-based singlecrystal substrate set forth in [6] to [8] below is provided so as toachieve the above object.

[6] A Ga₂O₃-based single crystal substrate, comprising a Ga₂O₃-basedsingle crystal comprising a donor impurity and Fe,

-   -   wherein a concentration of the Fe is higher than a concentration        of the donor impurity.

[7] The Ga₂O₃-based single crystal substrate according to [6], whereinthe donor impurity comprises Si.

The Ga₂O₃-based single crystal substrate according to [6] or [7],further comprising a principal surface that has a size and a shape largeenough to include a perfect circle of not less than 10 mm in diameter.

Advantageous Effects of the Invention

According to the invention, a Ga₂O₃-based single crystal substrate canbe provided which comprises a Ga₂O₃-based single crystal that has a highresistance while preventing a lowering of crystalline quality, as wellas a production method therefor.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows infrared-heating single crystal manufacturing equipment inan embodiment.

DESCRIPTION OF EMBODIMENTS Embodiment

(Ga₂O₃-Based Single Crystal Substrate)

A Ga₂O₃-based single crystal substrate in the present embodiment isformed of a Ga₂O₃-based single crystal which contains Fe as an acceptorimpurity in addition to a donor impurity such as Si such that the Feconcentration is higher than the donor impurity concentration.Therefore, the Ga₂O₃-based single crystal substrate in the presentembodiment has high electrical resistance.

The principal surface of the Ga₂O₃-based single crystal substratepreferably has a size and a shape which are enough to include a perfectcircle of not less than 10 mm in diameter. This size of the Ga₂O₃-basedsingle crystal substrate is suitable for mass production. Typicalexamples include a square of not less than 10 mm in each side, a perfectcircle of not less than 10 mm in diameter, a rectangle having shortsides of not less than 10 mm, and an ellipse having a minor axis of notless than 10 mm.

(Manufacture of Ga₂O₃-Based Single Crystal Substrate)

The Ga₂O₃-based single crystal substrate in the present embodiment iscut from a Ga₂O₃-based single crystal containing Fe which is doped as anacceptor impurity.

The Ga₂O₃-based single crystal in the present embodiment is a Ga₂O₃single crystal, or a Ga₂O₃ single crystal doped with elements such as Aland In. It may be, e.g., a (Ga_(x)Al_(y)In_((1-x-y)))₂O₃ (0<x≤1, 0≤y≤1,0<x+y≤1) single crystal which is a Ga₂O₃ single crystal doped with Aland In. The band gap is widened by adding Al and is narrowed by addingIn.

By using Fe as an acceptor impurity, it is possible to dope a sufficientamount of acceptor while inhibiting a reduction in crystal quality, andthereby possible to grow a high-resistivity Ga₂O₃-based single crystal.

The method of growing a Ga₂O₃-based single crystal is not limited to aspecific method and is, e.g., a FZ method, an EFG (Edge-defined Film-fedGrowth) method or a CZ (Czochralski) method, etc. The reason why themethod of growing a Ga₂O₃-based single crystal is not limited isconsidered that the effect of allowing a high-resistivity Ga₂O₃-basedsingle crystal to grow while inhibiting a reduction in crystal qualityis based on a solid solubility limit of Fe in a Ga₂O₃-based singlecrystal or the level of vapor pressure.

If a Ga₂O₃-based raw material having a purity of 99.999 mass % is usedto grow the Ga₂O₃-based single crystal, Fe is added to the Ga₂O₃-basedraw material such that the Fe concentration in the grown crystal is notless than 5×10¹⁷ cm⁻³. To achieve this, Fe is added in an amount of,e.g., not less than 0.001 mol %. Thereby, in the grown Ga₂O₃-basedsingle crystal, the Fe concentration is higher than the concentration ofSi which is derived from the Ga₂O₃-based raw material and functions as adonor impurity.

If a Ga₂O₃-based raw material having a purity of 99.99 mass % is used togrow the Ga₂O₃-based single crystal, Fe is added to the Ga₂O₃-based rawmaterial such that the Fe concentration in the grown crystal is not lessthan 5×10¹⁸ cm⁻³. To achieve this, Fe is added in an amount of, e.g.,not less than 0.01 mol %. Thereby, in the grown Ga₂O₃-based singlecrystal, the Fe concentration is higher than the concentration of Siwhich is derived from the Ga₂O₃-based raw material and functions as adonor impurity.

Here, when the Ga₂O₃-based single crystal is, e.g., a Ga₂O₃ singlecrystal, the Ga₂O₃-based raw material is Ga₂O₃ powder. Meanwhile, whenthe Ga₂O₃-based single crystal is a (Ga_(x)Al_(y)In_((1-x-y)))₂O₃(0<x≤1, 0≤y≤1, 0<x+y≤1) single crystal, a mixture of Ga₂O₃ powder, Al₂O₃powder and In₂O₃ powder is used.

The Ga₂O₃-based single crystal in the present embodiment is, e.g., aβ-Ga₂O₃-based single crystal but may be a Ga₂O₃-based single crystalhaving another structure such as α-Ga₂O₃-based single crystal. Likewise,the Ga₂O₃-based single crystal substrate is, e.g., a β-Ga₂O₃-basedsingle crystal substrate but may be a Ga₂O₃-based single crystal havinganother structure such as α-Ga₂O₃-based single crystal substrate.

Next, a method using FZ technique will be described as an example of theproduction method for a Ga₂O₃-based single crystal substrate.

FIG. 1 shows infrared-heating single crystal manufacturing equipment inthe embodiment. The infrared-heating single crystal manufacturingequipment 10 is to grow a Ga₂O₃-based single crystal 5 using FZtechnique, and has a quartz tube 11, a seed chuck 12 for holding a seedcrystal 2 formed of a Ga₂O₃-based single crystal, a vertically-movablelower rotating shaft 13 to transmit rotation to the seed chuck 12, a rawmaterial chuck 14 for holding a polycrystalline material 3 formed of aGa₂O₃-based polycrystal, a vertically-movable upper rotating shaft 15 totransmit rotation to the raw material chuck 14, and an oval mirror 17which houses halogen lamps 16 and collects light emitted from thehalogen lamps 16 to a predetermined position of the polycrystallinematerial 3.

The quartz tube 11 houses the seed chuck 12, the lower rotating shaft13, the raw material chuck 14, the upper rotating shaft 15, the seedcrystal 2, the polycrystalline material 3 and the Ga₂O₃-based singlecrystal 5. A mixture gas of an oxygen gas and a nitrogen gas as an inertgas is supplied into the quartz tube 11 and is hermetically-sealedtherein.

An upper edge of the seed crystal 2 is brought into contact with a loweredge of the polycrystalline material 3 by adjusting a vertical positionof the upper rotating shaft 15, and in this state, a contact portiontherebetween is heated and melted by collecting light of the halogenlamp 16 thereto. Then, the heated portion is moved by pulling thepolycrystalline material 3 upward while appropriately rotating both orone of the seed crystal 2 and the polycrystalline material 3, therebygrowing the Ga₂O₃-based single crystal 5 to which crystal information ofthe seed crystal 2 is transferred.

In FIG. 1 which shows the Ga₂O₃-based single crystal 5 in the middle ofgrowth, the upper side of a melted portion 4 melted by heat is thepolycrystalline material 3 and the lower side is the Ga₂O₃-based singlecrystal 5.

Next, a specific process of growing a Ga₂O₃ single crystal as theGa₂O₃-based single crystal 5 using the infrared-heating single crystalmanufacturing equipment 10 will be described.

Firstly, the seed crystal 2, which is formed of a β-Ga₂O₃ singlecrystal, and the polycrystalline material 3, which is formed of aFe-containing β-Ga₂O₃ polycrystal produced by adding Fe to Ga₂O₃ powderhaving a purity of 99.999 mass %, are prepared separately. Here, it ispossible to use pure Fe or Fe oxide as a raw material of Fe to be addedto the Ga₂O₃ powder.

Next, in the quartz tube 11, the seed crystal 2 is bought into contactwith the polycrystalline material 3 and the contact portion is heatedsuch that both the seed crystal 2 and the polycrystalline material 3 aremelted at the contact portion. Once the molten polycrystalline material3 is crystallized together with the seed crystal 2, a Ga₂O₃ singlecrystal as the Ga₂O₃-based single crystal 5 containing Fe is producedabove the seed crystal 2.

Here, the grown Ga₂O₃ single crystal as the Ga₂O₃-based single crystal 5has a size such that a Ga₂O₃ single crystal substrate with a principalsurface having a size and a shape enough to include a perfect circle ofnot less than 10 mm in diameter can be cut out.

Next, the Ga₂O₃ single crystal is processed by cutting, etc., therebyobtaining a high-resistivity Ga₂O₃ single crystal substrate.

When Fe was added in an amount of 0.01 mol % and 0.05 mol %, cracks werenot generated on the Ga₂O₃ single crystal in both cases and aGa₂O₃-based single crystal substrate having a square principal surfaceof not less than 10 mm in each side was obtained.

The Fe concentration in the obtained Ga₂O₃ single crystal was about5×10¹⁸ cm⁻³ when adding 0.01 mol % of Fe and was about 1.5×10¹⁹ cm⁻³when adding 0.05 mol % of Fe. Resistivity of the Ga₂O₃ single crystalsubstrate with 0.05 mol % of Fe was about 2×10¹² Ωcm.

For the purpose of comparison with the present embodiment, an elementother than Fe was doped as an acceptor impurity into the Ga₂O₃ singlecrystal as Comparative Example. The experimental result is describedbelow. The experimental conditions, except an acceptor impurity to bedoped, were the same as those for the above-mentioned test using Fe fordoping.

When the Ga₂O₃ single crystal was grown with Mg added at 0.05 mol %,cracks were easily generated on the grown crystal and it was notpossible to cut out a Ga₂O₃ single crystal substrate having a squareprincipal surface of not less than 10 mm in each side. It is consideredthat this is because the solid solubility limit of Mg in the Ga₂O₃single crystal is lower than that of Fe.

Based on this result, another Ga₂O₃ single crystal was then grown withMg added at 0.01 mol %. As a result, generation of cracks was reducedand a substrate having a suitable size for mass production was cut outbut the substrate did not exhibit high resistivity. The Mg concentrationin the substrate was about 2 to 5×10¹⁷ cm⁻³ and was less than the Siconcentration which was about 5×10¹⁷ cm⁻³ in the high concentrationregion. From this result, it was found that it is necessary to useexpensive Ga₂O₃ powder having a purity of not less than 99.9999% andhaving a lower Si concentration when Mg is doped to increase resistivityof the Ga₂O₃ single crystal without generation of cracks.

Meanwhile, when 0.05 mol % of Zn was added to the Ga₂O₃ powder for thepurpose of doping Zn into the Ga₂O₃ single crystal, Zn was evaporatedduring calcination for making a rod-shaped polycrystalline material andthe grown Ga₂O₃ single crystal did not exhibit high resistivity. The Znconcentration in the grown Ga₂O₃ single crystal analyzed by SIMS(secondary ion mass spectrometry) was below the lower detection limit(not more than 1×10¹⁷ cm⁻³). It was found from this result that it isdifficult to dope Zn which has a high vapor pressure.

Effects of the Embodiment

According to the embodiment, use of Fe as an acceptor impurity allows ahigh-resistivity Ga₂O₃-based single crystal to be grown while inhibitinga reduction in crystal quality thereof, and a Ga₂O₃-based single crystalsubstrate having a suitable size for mass production is obtained fromsuch a high-resistivity Ga₂O₃-based single crystal at low cost.

High-resistivity Ga₂O₃-based single crystal substrates can be used tomanufacture, e.g., Ga₂O₃-based transistors and use of the Ga₂O₃-basedsingle crystal substrate in the present embodiment thus allowsGa₂O₃-based transistors to be mass-produced. Since Ga₂O₃-basedtransistors are expected to have lower loss and higher breakdown voltagethan GaN-based transistors or SiC-based transistors which have beenbeing developed as next-generation power device materials, aglobal-scale significant energy-saving effect is expected if massproduction of Ga₂O₃-based transistors is achieved.

Although the embodiment of the invention has been described above, theinvention is not intended to be limited to the embodiment, and thevarious kinds of modifications can be implemented without departing fromthe gist of the invention.

For example, although Si is mentioned as an example of a donor impuritycontained in the Ga₂O₃-based single crystal in the embodiment, the donorimpurity is not limited to Si and may be a group IV element equivalentto Si. One electron is produced by substitution of a group IV elementequivalent to Si for a Ga atom in the Ga₂O₃-based single crystal,meaning that the group IV element equivalent to Si functions as a donorimpurity in the same manner as Si. Even in this case, the effects of theembodiment described above are obtained in the same manner as the casewhere Si is used as a donor impurity.

In addition, the invention according to claims is not to be limited tothe embodiment. Further, it should be noted that all combinations of thefeatures described in the embodiment are not necessary to solve theproblem of the invention.

INDUSTRIAL APPLICABILITY

The invention provides a Ga₂O₃-based single crystal substrate comprisinga Ga₂O₃-based single crystal that has a high resistance while preventinga lowering of crystalline quality, as well as a production methodtherefor.

REFERENCE SIGNS LIST

2 SEED CRYSTAL

3 POLYCRYSTALLINE MATERIAL

5 Ga₂O₃-BASED SINGLE CRYSTAL

The invention claimed is:
 1. A Ga₂O₃-based single crystal substrateconsisting of a Ga₂O₃-based single crystal comprising a donor impurityand Fe, wherein a concentration of the Fe is higher than a concentrationof the donor impurity to increase a resistance value of the substrate,and wherein the donor impurity is Si, and the Fe concentration in theGa₂O₃-based single crystal is not less than 0.001 mol % and is notgreater than 0.05 mol %.
 2. The Ga₂O₃-based single crystal substrateaccording to claim 1, further comprising a principal surface that has asize and a shape large enough to include a perfect circle of not lessthan 10 mm in diameter.
 3. The Ga₂O₃-based single crystal substrateaccording to claim 1, wherein the substrate resistivity is about 2×10¹²Ωcm.
 4. The Ga₂O₃-based single crystal substrate according to claim 1,wherein the Fe concentration is 0.01 to 0.05 mol %.